This invention concerns a strata movement indicator.
The present invention has particular, but not exclusive, reference to an indicator for use in underground mines, for example coal mines, in association with rock bolted strata.
The rock bolting of strata defining roadways or galleries in underground mines has long been known and applied, dependent upon the nature of the strata and the method of working the mineral seams. Although it has found widespread application for example in the United States, it has not been until more recent times that it has found favor in the United Kingdom and Europe where more conventional supports, for example arches, have predominated. By virtue of their distortion or displacement, such supports give a readily observable indication as to the manner in which movement within the surrounding strata is taking place.
However, when employing rock bolts alone for reinforcement, because they are embedded within the strata there is no readily external visual indication as to the movement of the strata.
In assignee's co-pending British Patent Application No 91 05296.9 we describe a simple yet effective strata movement indicator which includes the features of an anchor at the remote end of a borehole and a reference tube at the near end of the same borehole. The anchor is connected to a banded indicator by means of a wire. Expansion of the strata causes the indicator to move relative to the reference tube by an amount equal to the net expansion between the anchor and the reference tube. While the indicator performs satisfactorily, it is incapable of focusing upon the source of the strata movement.